The present invention relates to an ultrasonic motor, and more particularly to an ultrasonic motor having reduced dimensions in an axial direction and a radial direction, thus to be made smaller and thinner.
An ultrasonic motor includes a stator constituted of a comb teeth body circumferentially disposed in close contact with a piezoelectric element and integrally formed therewith, and a disk-shaped or ring-shaped rotor rotatably butted to the stator at a predetermined pressure, and is configured to apply a high frequency voltage to the piezoelectric element of the stator to cause the piezoelectric element to vibrate, and to circumferentially expand the vibration with the comb teeth body integrally provided with the piezoelectric element for circumferentially moving the comb teeth body in a traveling wave form, thus to cause the rotor, which is frictionally engaged with the piezoelectric element, to rotate around the shaft. Accordingly, a mechanism that presses the rotor into contact with the stator is required, and conventionally a spring means has been proposed as the pressure source for the ultrasonic motor.
FIG. 7 is a cross-sectional view showing an example of such a conventional ultrasonic motor, which includes a flat disk-shaped piezoelectric element 12 circumferentially polarized in a plurality of poles and integrally mounted on a disk-shaped base 14 with fixing holes for the motor, and a short cylindrical container-shaped comb teeth body 11 with a plurality of comb teeth 116 circumferentially aligned integrally provided on the piezoelectric element 12, and the piezoelectric element 12 and the comb teeth body 11 constitute a stator 1. To the piezoelectric element 12, a high frequency voltage is to be applied via a flexible substrate 13. At the center of the base 14 a shaft hole 141 is provided, in which a cylindrical sleeve 142 is fixed along the inner circumferential surface thereof. Inside the sleeve 142 a ball bearing 15 is enclosed, so as to pivotally support a rotating shaft 3A. To the rotating shaft 3A, a short cylindrical shaped rotor 2 is attached, such that an end face of a peripheral wall portion 22 extended from the peripheral edge of the rotor 2 is butted to each of the comb teeth 116 of the comb teeth body 11. Also, a compressed coil spring 143 is inserted in an axial direction between an upper end of the sleeve 142 according to the drawing and the ball bearing 15, so that the axial elastic force of the compressed coil spring 143 biases the ball bearing 15 and the supporting rotating shaft 3A downward according to FIG. 7, thus to press the lower end face of the peripheral wall portion 22 of the rotor 2 to the upper end face of the comb teeth 116 of the stator 1.
In this ultrasonic motor, when a high frequency voltage is applied to the piezoelectric element 12 via the flexible substrate 13, the piezoelectric element 12 vibrates, and hence the comb teeth body 11 integrally mounted therewith vibrates, to thereby circumferentially displace the comb teeth 116 which are circumferentially aligned. Accordingly, the peripheral wall portion 22 of the rotor 2 butted to the comb teeth 116 is equally moved circumferentially by the frictional force, so that the rotor 2 and the rotating shaft 3A integrally attached thereto are caused to rotate. The rotational force of the rotating shaft 3A is transmitted to outside via a gear 16 attached to the rotating shaft 3A.
Thus, the coil spring 143 is employed as the elastic source to press the rotor 2 against the stator 1 at a required pressure, however the coil spring 143 has to have a predetermined axial length in order to attain the necessary pressure, which inhibits making the axial dimension of the ultrasonic motor shorter than the length of the coil spring 143. Besides, to facilitate the transmission of the vibration of the comb teeth body 11 to the rotor 2, the contact surface of the comb teeth body hand the contact surface of the rotor 2 have to remain precisely parallel, in other words the rotating shaft 3A has to remain precisely perpendicular with respect to the contact surface of the rotor 2 and the contact surface of the comb teeth body 11. However, since those components are each independent, high processing precision is required. Accordingly, it is preferable that the rotating shaft 3A is granted with certain freedom to tilt with respect to the central axis, to alleviate the precision requirement. This requires that the axial dimension between the rotor 2 and the ball bearing 15 be made longer than the necessary length. Consequently, it is difficult to design a thin ultrasonic motor with a reduced axial dimension, from the ultrasonic motor of the structure as shown in FIG. 7. Besides, although the ball bearing 15 is employed to pivotally support the rotating shaft 3A, a small ball bearing suitable for an ultrasonic motor is generally expensive, and therefore employing the ball bearing impedes achieving an ultrasonic motor at a low cost.
Regarding the foregoing problem, Japanese Patent Provisional Publication No. P2000-60154A (hereinafter, referred to as '154 publication) proposes an ultrasonic motor in which a circular disk portion surrounded by a peripheral wall portion of a short cylindrical-shaped rotor is formed as a thin coned disk spring, so as to utilize the spring force provided by the disk portion to press the peripheral wall portion of the rotor against the comb teeth portion of the stator. The ultrasonic motor according to '154 publication eliminates the need to employ the coil spring, and is therefore advantageous in making the ultrasonic motor thinner.
In the ultrasonic motor according to '154 publication, the spring force is created by the disk portion of the rotor, however making the disk portion thicker thus to increase the spring force degrades the deformability of the peripheral wall portion, which leads to insufficient contact between the rotor and the comb teeth. On the contrary, making the disk portion thinner to secure proper contact between the rotor and the comb teeth results in reduced spring force. In this case, besides, a predetermined displacement amount of the peripheral wall portion has to be secured, for which the disk portion has to have a certain diameter. Accordingly, the radial dimension of the disk portion cannot be made smaller than a certain limit, and it is therefore difficult to make the ultrasonic motor radially smaller. On the other hand, since the disk portion is attached to a rotating shaft via a ball bearing, the ball bearing cannot be omitted, which makes it difficult to reduce the cost.